1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method.
2. Description of the Related Art
Typically, a color image forming apparatus includes a plurality of image forming units for forming a multicolor image. The image forming apparatus adjusts a registration in a sub-scanning direction at the timing to start an image forming operation with a system operation start signal generated based on a signal detected by a registration sensor. The registration sensor detects a position of a recording medium such as a printing sheet and outputs the start signal. As such color image forming apparatus, for example, a tandem-type color image forming apparatus is known, in which image forming units, such as photosensitive elements, for respective colors are aligned along a conveying belt so called an intermediate transfer belt. In the tandem-type color image forming apparatus, images of respective colors formed by the image forming units are superimposed at the same position of the intermediate transfer belt and the superimposed color images are transferred onto a printing sheet by a transferring unit. The intermediate transfer belt is an endless belt supported by a plurality of rollers with a predetermined tension, and is driven to move at a constant speed in a predetermined direction by a drive unit that rotates a driving roller.
In such a tandem-type color image forming apparatus, each image forming unit starts image formation at a different timing, so that an operation start signal for the entire system is generated based on a signal detected by a sensor, such as the registration sensor, that detects a position of a printing sheet. Then, timing to start an image forming operation is adjusted for each color based on the operation start signal, thereby changing a registration amount in the sub-scanning direction.
For example, Japanese Patent Application Laid-open No. 2005-178080 discloses a technology for controlling color registration in the sub-scanning direction in units of line in a tandem-type color image forming apparatus by the following method. That is, in a write control unit as an image forming unit, a system operation start signal that is a perfect asynchronous operation signal is controlled in units of line cycle by a synchronous detection signal that is a reference signal of the write control unit, and a reference position of starting sub-scanning for each color is timing controlled by the synchronous detection signal.
On the other hand, in recent years, a technology is put into practical use for improving a printing speed, in which a plurality of lines (e.g., 8 to 10 lines) is scanned in one scanning by using an optical deflector such as a polygon mirror to form an electrostatic latent image in units of a plurality of lines in each scanning.
The technology disclosed in Japanese Patent Application Laid-open No. 2005-178080 enables control of color registration in units of scanning in the sub-scanning direction. However, in a color image forming apparatus that forms a latent image in units of a plurality of lines in each scanning, a position in the sub-scanning direction cannot be controlled in unit of line in one scanning.